Singer Chris Rea is said to be in a stable condition after collapsing during a performance.
Fans posted on Twitter that they saw the 66-year-old “fall backwards” on stage at the New Theatre Oxford.
South Central Ambulance Service said it was called at 21:30 GMT to an incident at the venue and that one patient had been taken to hospital.
Rea, who is known for his hits “Driving Home for Christmas” and “Road to Hell”, is on tour to promote his new album.
Darren Fewins, who was in the audience, said the musician from Middlesbrough had collapsed halfway through a song.
He told the Press Association that Rea was “playing the best I have ever seen” before his fall.
Staff at the theatre tweeted that they “appreciate everyone’s patience” and will update people with news when they have it.
Rea suffered a stroke in 2016, but recovered to launch an album in September and embark on his tour around Europe from October.
He is due to play two more dates in Brighton and Bournemouth.